tools: fix TestFdtUtil()

Variable old_outdir cannot be used before assignment.
The assignment must occur before the try block.

tools/dtoc/test_fdt.py:796:26:
E0601: Using variable 'old_outdir' before assignment
(used-before-assignment)

Add missing space in assignment.

Fixes: a004f29464d1 ("binman: Tidy up _SetupDtb() to use its own temporary file")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/dtoc/test_fdt.py b/tools/dtoc/test_fdt.py
index 32fa69c..4fe8d12 100755
--- a/tools/dtoc/test_fdt.py
+++ b/tools/dtoc/test_fdt.py
@@ -784,8 +784,8 @@
 
     def test_ensure_compiled_tmpdir(self):
         """Test providing a temporary directory"""
+        old_outdir = tools.outdir
         try:
-            old_outdir = tools.outdir
             tools.outdir= None
             tmpdir = tempfile.mkdtemp(prefix='test_fdt.')
             dtb = fdt_util.EnsureCompiled(find_dtb_file('dtoc_test_simple.dts'),
@@ -793,7 +793,7 @@
             self.assertEqual(tmpdir, os.path.dirname(dtb))
             shutil.rmtree(tmpdir)
         finally:
-            tools.outdir= old_outdir
+            tools.outdir = old_outdir
 
     def test_get_phandle_name_offset(self):
         val = fdt_util.GetPhandleNameOffset(self.node, 'missing')